It has been recognized that it is desirable to apply aluminum oxide to thin film magnetic heads as a wear resistant coating by sputtering the dielectric onto the wafer. In the past, a sputtered aluminum oxide coating was usually applied by either radio frequency sputtering of aluminum oxide from a target composed of aluminum oxide or direct current reactive sputtering of aluminum from a metallic aluminum target through a sputtering atmosphere comprised of either pure oxygen or a mixture of oxygen and argon.
Both of these prior art processes, however, suffer from several drawbacks. Rates of deposition of coatings from a dielectric target such as aluminum oxide, are usually quite low. In addition the substrate or wafer receiving the coating usually becomes very warm. The prior art method of radio frequency magnetron deposition from an aluminum oxide target of aluminum oxide films at deposition rates exceeding 300 Angstroms per minute often yields substrate temperatures in excess of 300.degree. C. Electrons also bombard the substrate causing undesirable reactions in or damage to the film or underlying layers of the substrate. Another prior art process employs radio frequency sputtering of a metal target. However, in that process the cathode can become oxidized which reduces the rate of sputter deposition.
The prior art deposition of aluminum oxide from an aluminum oxide target usually employed pure argon as the sputtering gas. The dielectric films deposited through the pure argon are often deficient in oxygen and exhibit lower abrasion resistance and lower breakdown voltages than anticipated. When attempts are made to increase the electrical resistivity and the breakdown voltage of the deposited thin film by adding oxygen to the sputtering argon gas, the deposition rate from the aluminum oxide target is reduced to about one half of the deposition rate using pure argon. Furthermore, the deposited films exhibit high defect densities which are attributed to the accumulation of particulate aluminum oxide which flakes off shields and fixtures within the sputtering chamber.
In the alternative prior art process, which employs a metal target and reaction of the sputtered target material with partial pressures of reactive gases such as oxygen to form aluminum oxide, the sputtering apparatus can be operated at higher power densities per unit target area without failure of the target or metal bond since the metal of the target exhibits higher thermal conductivity and greater plasticity than the aluminum oxide targets. Furthermore, although it is necessary to employ a radio frequency sputtering apparatus for deposition from a dielectric target, a DC magnetron cathode can be employed to sputter conductive metal. The anode of the DC magnetron cathode can be arranged to collect free electrons in the sputtering chamber, thereby preventing the electrons from striking the substrate and damaging it. However, one of the drawbacks of this process is that the reactive gas, usually oxygen, also reacts with the clean metal surface of the sputtering target, causing a decrease in the sputtering rate from the target and considerable arcing and variations in load impedance. Since the prior art reactive sputtering process is often controlled by measuring electrical characteristics, such as target voltage or current, changes in the electrical characteristics of the target due to a build-up of a dielectric film, as a result of chemical reaction with oxygen gas in the chamber, perturb the control process and prevent the formation of a uniform dielectric film from run to run.
What is needed then is a process which will allow sputtering of a material from a metallic target but at the same time insure that the thin film deposited on the substrate is a high quality dielectric. It is also important that the process is repeatable and easily controlled.